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By Wendy Zukerman DNA sequencing has identified difficult-to-diagnose diseases in humans – the first time the technology has been used in a clinic. The technique, which decodes thousands of genes simultaneously, has been used in laboratories to uncover genes related to diseases since 2009. Now it has successfully moved to the clinic, where patients do not know what is wrong with them and may not know their family history of disease, and clinicians have few clues about which genes might be causing the problem. Mitochondrial diseases, which affect the way the body produces energy, are notoriously difficult to diagnose. Found in at least one in every 5000 people, the diseases often involve many genes, and symptoms vary across organs. For example, common manifestations can include blindness, seizures, slow digestion and muscle pain. Currently, diagnosing such disorders can take months or even years, and involves an invasive muscle biopsy. DNA sequencing technology may help to speed things up. Elena Tucker and colleagues from the Murdoch Childrens Research Institute in Sydney, Australia, along with Vamsi Mootha from Harvard Medical School, sequenced the genomes of 42 children who had traits that suggested they carry a mitochondrial disorder. To work out exactly which disorder each child carries, the team looked both at the DNA in their mitochondria and at the 100 or so genes within their nuclear DNA that have already been linked to mitochondrial diseases. They also looked at a further 1000 nuclear genes that play a part in mitochondrial biology. To distinguish between harmless genetic variations and those that might cause a disease, the team compared the patients’ genomes with databases of genetic variation recorded in the general population. Ten of the children had mutations in genes previously linked to mitochondrial diseases, and so could be given a precise diagnosis. Mutations not previously associated with any disease were found in another 13 children. Tucker says that these patients can expect a full diagnosis once studies confirm the function of these genes. “We are quite excited,” says Tucker. “Most of these diagnoses were in children whose [illnesses] could not easily be diagnosed using traditional methods.” Michael Ryan, a biochemist at La Trobe University in Melbourne, Australia, who was not involved in the work, says the diagnosis rate “will improve” within the next couple of years as the list of genes known to be linked to mitochondrial diseases grows, and it becomes clearer how mutations combine to cause diseases. “It’s a fantastic study,” says Matthew McKenzie at Monash University in Melbourne. Finding genetic mutations in mitochondrial patients is “like searching for a needle in a haystack”, he says. “I think it was a very good result to transfer to a clinical setting.” Journal reference: Science Translational Medicine, DOI: 10.1126/scitranslmed.3003310 More on these topics: